JPS6244864B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to techniques for converting solar energy into electrical energy by silicon solar cells, and more particularly to thick film paste compositions for making low resistance contacts in such cells. It is well known that radiation of a suitable wavelength impinging on the PN junction of a semiconductor body serves as an external energy source for generating hole-electron pairs in the body. Because of the potential difference that exists at the P-N junction, holes and electrons move across the junction in opposite directions, thereby creating a current flow that can provide an output to an external circuit. Most solar cells exist in the form of silicon wafers with metal contacts that are metallized, ie conductive. In order to provide a low cost method of generating current from the P-N junction of silicon wafers, it is common to metallize the wafers by screen printing and baking operations. Commercially available metallizing inks used to deposit contacts on the surface of a wafer generally contain metal powder, micronized glass frit, and an organic vehicle. Typical metal powders are powders of silver, aluminum, nickel, gold or copper or their alloys with noble metals such as platinum and palladium. Si ₃ N ₄ is widely used in solar cell technology as an antireflective coating that also serves as a masking protective layer. It has good adhesion and stability when deposited on silicon. In a specific embodiment, a silicon solar cell is coated with Si _{3 N 4 on the front N-type side as an anti-reflective coating and during the process also coated with Si 3 N 4 on the} back P-type side. It becomes like this. An etching process must be employed to make electrical contact to the underlying silicon substrate. The state of the art requires that the Si ₃ N ₄ be removed in the area where the contacts are made, the front side being etched in the form of a pattern for the application of front side contacts and the back side generally also etched. Also etched for large area backside contact applications. If this etching step could be omitted, it would result in cost savings. It has now been found, according to the invention, that an Al-Mg alloy can perform this function when included in the metallization paste formulation. For example, when terminating Si ₃ N ₄ coated solar cells with base metals such as Ni-Sb alloys or aluminum, the incorporation of 50Al:50Mg alloy powder into the metallization paste results in improvements in electrical properties and during sintering. The firing window, ie the temperature range for sufficient discharge initiation, is widened. In a specific embodiment, the invention provides a P-type moiety and an N
- in thick film metallization pastes used to provide low resistance conductive contacts (terminations) for Si ₃ N ₄ coated silicon solar cells with mold sections and P/N junctions; In fact, this paste contains a large proportion of metal powder (e.g. aluminum or Ni-
Sb alloy), a small proportion of micronized glass frit and a small amount of a mixture of 50Al:50Mg alloy in particle form in an organic vehicle. The invention further relates to a method of metallizing batteries and the resulting products. The metallization process typically consists of screen printing one surface of the cell with the metallization paste of the invention and firing at a temperature of at least 500°C. The invention is illustrated by the following examples. Example 1 Pyramidal crystal structure on the diffusion side
P etched to generate texture)
-N- type silicon wafer with a depth of 0.4~0.5μ
constructed by applying type impurities and
A front side textured silicon solar cell with a Si ₃ N ₄ anti-reflective coating was metallized and provided with metal contacts or terminals. A metallization paste was screen printed onto the N-surface of the wafer and consisted of an organic vehicle (ethylcellulose/dibutyl phthalate in terpineol), NiSb alloy, glass frit and a small amount of 50Al:50Mg alloy. The composition of glass frit is
They were 83% by weight of PbO, 4.9% by weight of PbF ₂ , 11% by weight of B ₂ O ₃ and 1.1% by weight of SiO ₂ . Three paste samples were prepared and the terminals were fired in a nitrogen atmosphere.
The proportions of metal components were varied as shown in the table below.

[Table] The soldered electrical characteristics of nitrogen-fired solar cells are listed in the table.

[Table] The table shows that terminals not containing the 50Al:50Mg alloy have a relatively low sintering temperature onset region near 550°C, but this onset region extends to at least 500°C when the alloy is present. It shows. The table also shows the electrical properties of the solar cell, namely the series conductivity and the Voc (voltage developed across the cell when irradiated by the sun, although no current flows through the cell) when using this alloy. It also shows that it can be enhanced. This is 50Al:
50Mg alloy penetrates _{Si3N4 coating} and N
- Shows effectiveness in making contact with the mold area. EXAMPLE 50Al:50Mg alloy was incorporated into two out of three samples of thick film aluminum-based metallization paste and these were screen printed onto the P-type back surface of a Si ₃ N ₄ coated silicon solar cell. Applied. The resulting terminal was fired in air. The table shows the reverse contact resistance in ohms for solar cells terminated in this way (using two probe measurements between the parallel conductors on the P surface of the silicon cell).

[Table] From the table, it is clear that the contact resistance of the formulation when 50Al:50Mg is added is significantly reduced compared to the case without 50Mg. The above embodiments are merely illustrative. Other vehicles, other metal powders, other glass frit compositions and other Al:Mg alloys are also available as they may be used to provide low resistance conductive contacts to _Si3N4 coated silicon solar cells _. It should be understood that it can be used as long as it helps produce a thick film metallization paste. Although screen printing is disclosed above, for example, brush painting,
Other substrate application methods such as spraying, stamping, etc. may also be used. The organic vehicle used in the printing paste is generally
Amounts are used to contain ~90% fixation and 10-30% vehicle. A number of inert liquid vehicles commonly used in the art are described in U.S. Pat.
It is described in detail in column 4, lines 3 to 28 of specification No. 4172919.

Claims (1)

[Claims] 1. Si ₃ N, characterized in that it consists of an organic vehicle containing a major amount of metal powder, a minor amount of micronized glass frit and a small amount of 50Al:50Mg alloy in particle form. Metallization paste for providing conductive contacts on silicon solar cells coated with ₄ . 2. The metallizing paste according to claim 1, wherein the metal powder is Al powder. 3. The metallizing paste according to claim 1, wherein the metal powder is a NiSb alloy powder.